Understanding Contrast (Minimum)

SC text

Intent of Contrast (Minimum)

The intent of this Success Criterion is to provide enough contrast between text and
its background so that it can be read by people with moderately low vision (who do
not use contrast-enhancing assistive technology). For people without color deficiencies,
hue and saturation have minimal or no effect on legibility as assessed by reading
performance (Knoblauch et al., 1991). Color deficiencies can affect luminance contrast
somewhat. Therefore, in the recommendation, the contrast is calculated in such a way
that color is not a key factor so that people who have a color vision deficit will
also have adequate contrast between the text and the background.

Text that is decorative and conveys no information is excluded. For example, if random
words are used to create a background and the words could be rearranged or substituted
without changing meaning, then it would be decorative and would not need to meet this
criterion.

Text that is larger and has wider character strokes is easier to read at lower contrast.
The contrast requirement for larger text is therefore lower. This allows authors to
use a wider range of color choices for large text, which is helpful for design of
pages, particularly titles. 18 point text or 14 point bold text is judged to be large
enough to require a lower contrast ratio. (See The American Printing House for the
Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large
Print under
Resources). "18 point" and "bold" can both have different meanings in different fonts but,
except for very thin or unusual fonts, they should be sufficient. Since there are
so many different fonts, the general measures are used and a note regarding fancy
or thin fonts is included.

When evaluating this success criterion, the font size in points should be obtained
from the user agent or calculated on font metrics in the way that user agents do.
Point sizes are based on the CSS pt size as defined in
CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt
and 18pt are equivalent to approximately 18.5px and 24px.

Because different image editing applications default to different pixel densities
(e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing
application can be unreliable when it comes to presenting text at a specific size.
When creating images of large-scale text, authors should ensure that the text in the
resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the
default size for body text. For example, for a 72 PPI image, an author would need
to use approximately 19 pt and 24 pt font sizes in order to successfully present images
of large-scale text to a user.

The previously-mentioned contrast requirements for text also apply to images of text
(text that has been rendered into pixels and then stored in an image format) as stated
in Success Criterion 1.4.3.

This requirement applies to situations in which images of text were intended to be
understood as text. Incidental text, such as in photographs that happen to include
a street sign, are not included. Nor is text that for some reason is designed to be
invisible to all viewers. Stylized text, such as in corporate logos, should be treated
in terms of its function on the page, which may or may not warrant including the content
in the text alternative. Corporate visual guidelines beyond logo and logotype are
not included in the exception.

In this provision there is an exception that reads "that are part of a picture that
contains significant other visual content,". This exception is intended to separate
pictures that have text in them from images of text that are done to replace text
in order to get a particular look.

Images of text do not scale as well as text because they tend to pixelate. It is also
harder to change foreground and background contrast and color combinations for images
of text, which is necessary for some users. Therefore, we suggest using text wherever
possible, and when not, consider supplying an image of higher resolution.

The minimum contrast success criterion (1.4.3) applies to text in the page, including
placeholder text and text that is shown when a pointer is hovering over an object
or when an object has keyboard focus. If any of these are used in a page, the text
needs to provide sufficient contrast.

Although this Success Criterion only applies to text, similar issues occur for content presented in charts, graphs, diagrams, and other non-text-based information.
Content presented in this manner should also have good contrast to ensure that more
users can access the information.

See also
.

Text that is larger and has wider character strokes is easier to read at lower contrast.
The contrast requirement for larger text is therefore lower. This allows authors to
use a wider range of color choices for large text, which is helpful for design of
pages, particularly titles. 18 point text or 14 point bold text is judged to be large
enough to require a lower contrast ratio. (See The American Printing House for the
Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large
Print under
Resources). "18 point" and "bold" can both have different meanings in different fonts but,
except for very thin or unusual fonts, they should be sufficient. Since there are
so many different fonts, the general measures are used and a note regarding fancy
or thin fonts is included.

When evaluating this success criterion, the font size in points should be obtained
from the user agent or calculated on font metrics in the way that user agents do.
Point sizes are based on the CSS pt size as defined in
CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt
and 18pt are equivalent to approximately 18.5px and 24px.

Because different image editing applications default to different pixel densities
(e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing
application can be unreliable when it comes to presenting text at a specific size.
When creating images of large-scale text, authors should ensure that the text in the
resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the
default size for body text as rendered by the browser.

Hues are perceived differently by users with color vision deficiencies (both congenital
and acquired) resulting in different colors and relative luminance contrasts than
for normally sighted users. Because of this, effective contrast and readability are
different for this population. However, color deficiencies are so diverse that prescribing
effective general use color pairs (for contrast) based on quantitative data is not
feasible. Requiring good luminance contrast accommodates this by requiring contrast
that is independent of color perception. Fortunately, most of the luminance contribution
is from the mid and long wave receptors which largely overlap in their spectral responses.
The result is that effective luminance contrast can generally be computed without
regard to specific color deficiency, except for the use of predominantly long wavelength
colors against darker colors (generally appearing black) for those who have protanopia.
(We provide an advisory technique on avoiding red on black for that reason). For more
information see [[ARDITI-KNOBLAUCH-1994]]
[[ARDITI-KNOBLAUCH-1996]]
[[ARDITI]].

Some people with cognitive disabilities require color combinations or hues that have
low contrast, and therefore we allow and encourage authors to provide mechanisms to
adjust the foreground and background colors of the content. Some of the combinations
that could be chosen may have contrast levels that will be lower than those found
in the Success Criteria. This is not a violation of this Success Criteria provided
there is a mechanism that will return to the default values set out in the Success
Criteria.

A contrast ratio of 3:1 is the minimum level recommended by [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] for standard text and vision. The 4.5:1 ratio is used in this provision to account
for the loss in contrast that results from moderately low visual acuity, congenital
or acquired color deficiencies, or the loss of contrast sensitivity that typically
accompanies aging.

The rationale is based on a) adoption of the 3:1 contrast ratio for minimum acceptable
contrast for normal observers, in the ANSI standard, and b) the empirical finding
that in the population, visual acuity of 20/40 is associated with a contrast sensitivity
loss of roughly 1.5 [[ARDITI-FAYE]]. A user with 20/40 would thus require a contrast ratio of 3 * 1.5 = 4.5 to 1. Following
analogous empirical findings and the same logic, the user with 20/80 visual acuity
would require contrast of about 7:1.

The contrast ratio of 4.5:1 was chosen for level AA because it compensated for the
loss in contrast sensitivity usually experienced by users with vision loss equivalent
to approximately 20/40 vision. (20/40 calculates to approximately 4.5:1.) 20/40 is
commonly reported as typical visual acuity of elders at roughly age 80. [[GITTINGS-FOZARD]]

The contrast ratio of 7:1 was chosen for level AAA because it compensated for the
loss in contrast sensitivity usually experienced by users with vision loss equivalent
to approximately 20/80 vision. People with more than this degree of vision loss usually
use assistive technologies to access their content (and the assistive technologies
usually have contrast enhancing, as well as magnification capability built into them).
The 7:1 level therefore generally provides compensation for the loss in contrast sensitivity
experienced by users with low vision who do not use assistive technology and provides
contrast enhancement for color deficiency as well.

Calculations in [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] are for body text. A relaxed contrast ratio is provided for text that is much larger.

Conversion from nonlinear to linear RGB values is based on IEC/4WD 61966-2-1 [[IEC-4WD]] and on "A Standard Default Color Space for the Internet - sRGB" [[sRGB]].

The formula (L1/L2) for contrast is based on [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] standards.

The ANSI/HFS 100-1988 standard calls for the contribution from ambient light to be
included in the calculation of L1 and L2. The .05 value used is based on Typical Viewing
Flare from [[IEC-4WD]] and the [[sRGB]] paper by M. Stokes et al.

This Success Criterion and its definitions use the terms "contrast ratio" and "relative
luminance" rather than "luminance" to reflect the fact that Web content does not emit
light itself. The contrast ratio gives a measure of the relative luminance that would
result when displayed. (Because it is a ratio, it is dimensionless.)

Refer to
related resources for a list of tools that utilize the contrast ratio to analyze the contrast of Web
content.

See also
for techniques for indicating keyboard focus.

It is sometimes helpful for authors to not specify colors for certain sections of
a page in order to help users who need to view content with specific color combinations
to view the content in their preferred color scheme. Refer to
for more information.

Benefits of Contrast (Minimum)

People with low vision often have difficulty reading text that does not contrast with
its background. This can be exacerbated if the person has a color vision deficiency
that lowers the contrast even further. Providing a minimum luminance contrast ratio
between the text and its background can make the text more readable even if the person
does not see the full range of colors. It also works for the rare individuals who
see no color.